The present invention relates to turbines and, more particularly, to apparatus for guiding a sensor into the interior of a turbine when interior stationary and rotating elements therein are displaced for clearance restoration.
Gas and steam turbines extract energy from a high-temperature, high-pressure working fluid by impinging it on the blades or buckets of a plurality of rotatable turbine wheels. A set of stationary nozzles is positioned between each adjacent pair of turbine wheels. The nozzles are made up of a plurality of aerodynamic vanes which turn and accelerate the working fluid in order to impinge it on the buckets or blades of its following stage at the correct velocity and exit angle.
The outer extremities of the aerodynamic vanes are supported against the force of the working fluid by cantilevered attachment to an outer band, affixed to a turbine casing. The inner extremities of the aerodynamic vanes are affixed to an inner band. A diaphragm is affixed to the inner band radially inward thereof. The entire burden of supporting the vanes, diaphragm and inner band falls on the outer band since no stationary element may ride on the turbine shaft.
In a gas turbine, the outer band, vanes, inner band and diaphragm are called a nozzle assembly. In a steam turbine, the nozzle assembly is called the diaphragm.
In order to simplify the following description, the terminology of gas turbines will be used.
The high gas temperatures experienced by the vanes and diaphragms of the nozzle assembly are sufficient, when combined with high gas pressures, to force their material to deform in a downstream direction. In time, the material may creep sufficiently to alter clearances between rotating elements and contiguous elements of the nozzle assembly. If the creep is permitted to remain uncorrected, destructive rubbing may take place between stationary and rotating parts.
When clearances between stationary and rotating parts decrease a predetermined amount, it is conventional to remove the turbine wheel and nozzle assemblies, and to machine critical surfaces for restoring the clearances.
The machining operation generally entails shipment of the removed parts to a precision machine shop and a substantial delay in returning the turbine to service. Such precision machining is expensive. In addition, while the turbine is out of service, the work normally done by the turbine must either be foregone or be performed by alternate means which may entail the purchase of power and/or lease of substitute equipment. These solutions impose costs which are preferably avoided or reduced.
One alternative is to make at least some of the elements of the nozzle assembly of high-strength alloys capable of withstanding creep at the temperatures and forces involved. Such high-strength materials add significantly to the cost and are not repairable.
Another alternative employs additional air cooling of the affected parts. The cooling air added to the working fluid passing through the turbine undesirably degrades the efficiency of the turbine.
A further alternative, as disclosed in co-pending patent application Ser. No. 811,987), filed on the same date as the present application, provides means for displacing the nozzle assembly in the upstream direction sufficiently to restore clearances. When this is done, the connection point of a sensor guide tube, bridging the space between the turbine housing and interior elements of the turbine, is correspondingly shifted in the upstream direction. If means are not provided to correct it, such shifting of the connection point could result in an unacceptable bend in the passage for the sensor cable or a misalignment of the guide tube to the turbine housing port which would prevent assembly of the sensor into the guide tube.